Modifications of and chemical refinements to autocatalytic chemical reduction processes, in particular, electroless nickel (EN) plating solutions, are not uncommon. Many of these modifications address concerns related to the bath itself and its inherent properties of stability, plating rate and effective pH operating range for the plating environment. For example, U.S. Pat. No. 2,658,841 teaches the use of soluble organic acid salts as buffers for EN baths. U.S. Pat. No. 2,658,842 teaches the use of short chain, dicarboxylic acids as exaltants to EN baths. U.S. Pat. No. 2,762,723 teaches the use of sulfide and sulfur bearing additives to an EN bath for improved bath stability. U.S. Pat. No. 2,847,327 teaches the use of fatty acid compounds as stabilizers and mild exaltants for EN baths. This latter patent describes the use of numerous surfactants including organic compounds from the class of fatty acids and water-soluble salts thereof, amino compounds, and sulfates and sulfonates of fatty acids and fatty alcohols. What all of these patents have in common, is the use of a compound or class of compounds for the purpose of modifying the inherent properties of the plating bath itself (i.e. its plating rate, stability or useful pH operating range).
Further progress in autocatalytic plating since the U.S. Pat. No. 2,847,327 teaching has introduced other means of stabilizing an EN plating bath. These include the use of higher purity starting materials; more effective stabilizers from the class of heavy metals such as Pb, Sb, Bi, Cu and Se; inorganic compounds such as iodates, and thio compounds; organic compounds such as unsaturated alkenes and alkynes and others. Additionally, improvements in plating bath equipment, such as improved pumping and filtration methods and design, such as air sparging, improved methods of adding the replenishment chemistry to the plating tank and the use of anodic protection circuitry has further reduced concerns over bath stability. This invention is distinguished from U.S. Pat. No. 2,847,327 in that an additive is introduced into the plating bath for the purpose of improving the quality of the metal deposit by preventing, or at least very substantially inhibiting, co-deposition of non-metallic particles in the deposit. The function of the organic compounds in U.S. Pat. No. 2,847,327 is to act upon the bath solution. The function of the organic additive in this invention is to act upon the plated deposit not on the bath. Furthermore, not every organic compound taught in U.S. Pat. No. 2,847,327 will work in the practice of this invention, but only those having a sufficiently high zeta potential that it enables repulsion between the non-metallic, particles and the plating surface.
This invention has particular usefulness in the production of rigid memory disks that are quite commonly used in today's laptop and desktop computers. Details of the construction of thin film magnetic media are taught in U.S. Pat. No. 5,405,646. Media is built up in layers, each of which performs a specific task. The substrate of the disk can be glass, plastic, metal or any other rigid material. Commercially, both glass and aluminum have been widely used. The preferred practice for this invention is for an aluminum substrate. As shown in FIG. 1, an aluminum alloy, typically undergoes at least six wet chemical process steps to build up a hard, corrosion resistant, NiP coating layer. This serves as the underlayer for the subsequent application of magnetic media. It is this magnetic media which ultimately enables storage and deletion of data by electromagnetic currents produced and detected by read/write heads in today's hard-disk drives.
Prior to the application of magnetic media in the production of rigid memory disks, a few more treatments to the NiP coating are required. The plated disks are baked and polished. The polishing step produces an extremely flat and smooth surface for the subsequent sputtering steps and enables the very close fly heights (typically 30 nanometers) for the read/write heads in a finished hard-disk drive. Any slight aberrations or asperities (deviations from flatness) in the deposited coating (whether protrusions above or depressions below the otherwise flat surface) introduce susceptibilities to head crashes between the read/write heads and the surface of the hard-disk. This ultimately reduces the expected service life of these drive components.
It is in the electroless plating step where the present invention provides benefit by significantly reducing the potential for plating defects. During the wet chemical processing steps, the ground, aluminum disks, are racked on a plating fixture. Plating fixtures are very common in metal plating processes. The fixtures can be made from metal, plastic, glass or ceramics. The material chosen depends on many factors. In the production of memory disks, fixtures are commonly made from plastics. Engineering plastics are chosen from those that can withstand the heat and chemistry used in the electroless nickel plating process. These may include: fluorinated plastics, such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE); polysulfone (PSU); polyether ether ketone (PEEK) and polybisimidazole (PBI) among others.
Constant solution exchange at the surface of every disk is essential to refresh the plating chemistry at the surface and to produce a plated article of uniform and consistent composition which is of considerable importance in the memory disk industry. This is afforded by physical movement of both the plating fixture within the bath and vigorous solution mixing. The continuous mechanical movement of the plating fixture (on which the disks are mounted) can be translational, e.g., up and down or side-to-side, rotation and orbital motion of mounting spindles and mandrels, or that provided by some other means, e.g. ultrasonic motion. Solution movement or mixing can be provided by any type of fluid movement, e.g., due to recirculation pumps, cascading flow, jet nozzles, inductors, air sparging or any other means known to those in metal plating art and practice. The tank design can be of any physical shape, size or design as would be needed so that the parts can be contacted with the plating chemistry in such a manner that the metal required is built up and deposited on the object to be plated.
These different types of movement will cause the articles being plated to continually rub and potentially abrade small particles of plastic off the racking fixtures. These particles, having been created within the bath, or even introduced to the plating solution from some other source, be it internal or external, now have the possibility of being directed toward the plating surface. If this particle remains at the surface long enough, it now has the possibility of being encapsulated by the metal being deposited, i.e., built into the deposit and reducing the absolute purity of the coating due to the presence of an undesired, i.e. foreign, particle.
Due to the high degree of solution movement and mechanical motion within the plating tank, there is also a constant rubbing of the aluminum disks across the surface of the PVDF spindle and the polysulfone rod. This continual abrading action in a hot plating bath for nearly two hours at thousands of contact points between metal disks and plastic spindles and rods can cause small plastic particles to be detached and introduced into the plating solution. If these particles come in contact with a plating surface and remain there long enough, they can be plated into the growing NiP coating. Plated-in defects like these are known to occur and are of grave concern to production engineers. Once embedded, these particles can become exposed at the surface during the subsequent production step of polishing. A plastic particle located at the surface of the NiP layer can then become completely or partially dislodged when the final magnetic media is applied during the sputtering steps where heat (ca. 200-250° C.) is momentarily encountered.
When an asperity such as this is produced (i.e., a plastic protrusion or inclusion), hard disk reliability cannot be guaranteed because of the extremely low fly heights of the read/write heads over the surface of the spinning disk and the potential for head crashes. The head crash can be the result of either direct physical contact with the particle or a protrusion caused by it or due to a turbulent air flow pattern, as might be produced from a cavity or depression in the surface wherein a particle that once resided in the deposit as a foreign object has been produced due to a subsequent production step in the manufacture of the hard disk prior to the completed assembly of the final product. When this type of plating defect is found, even in just one disk from a plating batch of several thousand, the entire batch of disks is discarded and substantial losses are incurred.
To substantially avoid this type of inclusion, or vestige of its former presence, indicates that substantially no particles are detected in any article examined. In the hard disk industry “substantially” indicates no particles are allowed, i.e., the count frequency must be zero among those parts that are inspected. In other industries where particle dimension are even smaller than those encountered in the memory disk industry, e.g. some nano-engineering industry wherein foreign particles on a nanometer or picometer scale are of great commercial concern, the term “substantially” allows for a finite frequency of particles but it would be demonstrably, and statistically less than the number of foreign particles found when compared with a metal plating process by any commonly used statistical method for determining the purity (i.e., absence of the foreign particle) of the plated deposit.
One way of addressing this issue, relies on the high solution turnover and the use of in-line filtration to remove these particles. However, since the particles are generated within the plating tank, the possibility always remains that they may come into contact with the plating disks and remain there long enough to be encapsulated into the NiP coating.
Therefore, it is the object of the present invention to improve the quality of the plated deposit on a metal substrate with an autocatalytic chemistry. Of particular benefit is the application wherein a ground aluminum substrate is coated with an electroless nickel phosphorus alloy as in the manufacturing procedure used to produce rigid memory disks. As an enhancement to the present state of the art, the electroless nickel chemistry is modified with certain additives to nearly completely prevent the likelihood of co-depositing plastic particles in the coating.